As to Disjunctives, the attempt to put them through these different forms immediately destroys their disjunctive character.  Still, given any proposition in the form A is either B or C, we can state the propositions that give the sense of obversion, conversion, etc., thus: 

    DATUM.—­A is either B or C;
    OBVERSE.—­A is not both b and c;
    CONVERSE.—­Something, either B or C, is A;
    CONTRAPOSITIVE.—­Nothing that is both b and c is A.

For a Disjunctive in I., of course, there is no Contrapositive.  Given a Disjunctive in the form Either A is B or C is D, we may write for its Obverse—­In no case is A b, and C at the same time d.  But no Converse or Contrapositive of such a Disjunctive can be obtained, except by first casting it into the hypothetical or categorical form.

The reader who wishes to pursue this subject further, will find it elaborately treated in Dr. Keynes’ Formal Logic, Part II.; to which work the above chapter is indebted.

CHAPTER VIII

ORDER OF TERMS, EULER’S DIAGRAMS, LOGICAL EQUATIONS, EXISTENTIAL IMPORT OF PROPOSITIONS

Sec. 1.  Of the terms of a proposition which is the Subject and which the Predicate?  In most of the exemplary propositions cited by Logicians it will be found that the subject is a substantive and the predicate an adjective, as in Men are mortal.  This is the relation of Substance and Attribute which we saw (chap. i.  Sec. 5) to be the central type of relations of coinherence; and on this model other predications may be formed in which the subject is not a substance, but is treated as if it were, and could therefore be the ground of attributes; as Fame is treacherous, The weather is changeable.  But, in literature, sentences in which the adjective comes first are not uncommon, as Loud was the applause, Dark is the fate of man, Blessed are the peacemakers, and so on.  Here, then, ‘loud,’ ‘dark’ and ‘blessed’ occupy the place of the logical subject.  Are they really the subject, or must we alter the order of such sentences into The applause was loud, etc.?  If we do, and then proceed to convert, we get Loud was the applause, or (more scrupulously) Some loud noise was the applause.  The last form, it is true, gives the subject a substantive word, but ‘applause’ has become the predicate; and if the substantive ‘noise’ was not implied in the first form, Loud is the applause, by what right is it now inserted?  The recognition of Conversion, in fact, requires us to admit that, formally, in a logical proposition, the term preceding the copula is subject and the one following is predicate.  And, of course, materially considered, the mere order of terms in a proposition can make no difference in the method of proving it, nor in the inferences that can be drawn from it.